Hikaru and Kaoru Hitachiin
The twins Hikaru Hitachiin (常陸院 光 Hitachīn Hikaru?) and Kaoru Hitachiin (常陸院 馨 Hitachīn Kaoru?) are depicted as mischievous, devious and otherwise childish young men who toy with people, including their schoolmates. They were born on June 9, a date chosen by the manga artist to show that they are alike yet different, and are 15 years old at the beginning of the series.
When Hikaru and Kaoru were young, they were fond of their maid. One night, they discovered her trying to steal from their mansion and offered her a deal. She needed to guess which of the two Hikaru was in exchange for the combination to the safe. The maid took the combination number from them anyway and said that no one would ever be able to tell them apart. They eventually learned many skills from the maid, including how to fake emotions, before she fled the mansion and never returned.
Kaoru and Hikaru before joining the Host club, only trusted themselves. They did not have any other friends in school besides each other. In the "Which one is Hikaru Game", sometimes they wanted to be told apart, and sometimes they didn't. Haruhi is the only person that can tell the twins apart.
During their second year of junior high, they meet Tamaki Suou, who wants them to join the newly-formed Host Club. The twins agree to join only if Tamaki can guess which twin is which. Although Tamaki fails many times, he eventually guesses it, at which point the twins give in and decide to try the Host Club. The club opened them up to new experiences, including the development of a deep bond with Tamaki. Prior to joining, the twins described the world as "us" and "them", being very distrustful of and unkind to others. Additionally, because their parents were often away, they were deeply dependent on each other; joining the club and meeting Haruhi helped them become more independent.
The Hitachiin twins use acts of "forbidden brotherly love" as their main draw in the Host Club, with highly suggestive themes of incestuous boy love. They are both considered the mischievous, tricky, little devil types in the club. They have their differences: Kaoru is nicer, more mature and considerate to the thoughts of others, while Hikaru is more immature, even though he is the elder twin. Their different personalities are best shown when they are separated, and there is also a slight difference in the way they speak. Hikaru has a deeper voice and tends to talk first, while Kaoru voice is higher and a little more pitchy and he tends to talk second. There are several ways how to tell the twins apart, the most used being that Hikaru's hair is parted to the right side and Kaoru's hair parts to the left. And at the same time Hikaru tends to be on the left and Kaoru on the right. Hikaru has a crush on Haruhi.
The twins live with their mother, Yuzuha, a fashion designer, and their considerably meeker father, a computer software designer, until the final chapter in which they and the other Host Club members move to America to study with Haruhi.
In the volume 18 extras, they are shown around college age with their younger sister Ageha, born after the end of the series. She likes butterflies and flowers but has a personality much like her older brothers. She is attached to Tamaki, referring to him as milord/boss like Hikaru and Kaoru. The twins dote on her a lot. The twins eventually attend a college or university that is not Ouran University, since Ouran does not have a fine arts faculty.
Hikaru voiced by: Kenichi Suzumura (Japanese anime and later drama CDs), Soichiro Hoshi (early drama CDs), Shinpei Takagi (live-action), Todd Haberkorn (English)
Kaoru voiced by: Yoshinori Fujita (Japanese anime and later drama CDs), Kenichi Suzumura (early drama CDs), Manpei Takagi (live-action), Greg Ayres (English)